The present invention relates to an internal combustion engine and a method for operating it.
To hold open an inwardly opening, high pressure fuel injection solenoid valve used with gasoline direct injection, a holding phase is used, in which the current flowing through the high pressure injector is regulated with respect to an effective holding current value. The current regulation is carried out by an output stage of the electronic control unit, which results in a power loss that is a function of the flowing current. With large-scale integrated output stages in particular, the power loss can result in overheating and resultant failure of the output stage, which, in turn, can result in misfires. In these cases, the heat dissipation capability of the printed circuit board must be improved locally, which results in higher costs. The dispersion of the fuel injected by the high pressure injector is poorer the higher the holding current is, since the turn-off time of the current and, therefore, the valve closing time and the excess quantity injected depend on the level of the holding current. The level of the holding current is determined primarily by the maximum system pressure (against which the high pressure injector must be held open) and by the static flow rate.
The highest system pressure that exists during normal operation in systems with gasoline direct injection is determined via the opening of a pressure-limiting valve. The opening pressure of the pressure-limiting value is reached in two cases of normal operation. The first case is a hot start, i.e., a starting procedure after a shutoff phase, which is accompanied by an increase in pressure in the high pressure fuel system due to the fuel heating up. The fuel in the fuel system is heated up by the heat transferred from an engine that had been previously driven under full load and was therefore heated up to an extreme extent. The second case is the resumption of fuel injection after an overrun condition. In an overrun condition, fuel injection is halted, and pressure increases in the high pressure fuel system for the reason given above. In both cases, the pressure in the high pressure fuel system is lowered after a few injections to a normal, lower pressure level. The holding current is designed for the maximum attainable pressure, however, which is the opening pressure of the pressure-limiting valve. The power loss produced by the output stage and the dispersion of the fuel injected by the high pressure injector could be reduced if a reduced holding current were used. This is possible, in principle, except in the two cases mentioned above.
The object of the present invention is to make it possible for the valves to be held open reliably in all operating conditions.